Cathode circuit



April 17, 1934. J. B. MOORE CATHODE CIRCUIT Filed June 5, 1931 INVENTOR JOHN B.MOORE BY /f% 9 MM ATTORNEY Patented Apr. 17, 1934 CATHODE CIRCUIT John B. Moore, Riverhead, N. Y., assignor to Radio Corporation of America, a'corporation of Delaware Application June 5, 1931, Serial No. 542,251

3 Claims.

This invention relates to elec ron discharge device apparatus and is especially directed towards electron discharge devices having cathodes comprising a heated element and a heater eler ment.

In a system such as a high frequency radio receiving system utilizing a plurality of electron discharge devices it is desirable to operate the filaments or heaters of the several devices or tubes in series to conserve filament or heating current. Moreover, it is desirable, especially when direct or unidirectional current is applied to the heater elements, to bias cold electrodes of the devices such as control grids thereof from W the common filament or heater circuit. It is found, however, with simple routine connections for this purpose that the tubes so connected feed back energy over the common heating circuit from one to the other thereby causing undesirable self oscillations.

A primary object of my present invention is to prevent common coupling between various tuned circuits or cathode circuits associated with such a system.

To do so, it is an object of my present invention to provide a heating system for an electron discharge device or a plurality of such devices whereby feed back through the heater elements or leads supplying the heating filaments is substantially eliminated. Briefly, according to my nectingapath of high impedance to high frequencies between each electron emitting cathode and its corresponding heater element, and a path of low impedance to high frequencies from the electron emitting element or heated element to a point of substantially constant unidirectional potential and of substantially zero, or reference, high frequency potential, preferably ground. By doing so radio frequency or, more broadly, undulatory potentials on the heated element are effectively bypassed or grounded away from the heater element thereby preventing the flow of undulatory currents in the heating leads. In this manner the heated element is kept at practically the potential of the common or ground connection so far as high frequency or undulatory potentials are concerned, the ground connection being taken as the common reference point of zero potential.

Further, according to my present invention, by grounding a cold electrode of the device or devices, for unidirectional potentials, and maintaining the heated elements of the devices at a suitable unidirectional potential relative to the 55 cold electrode, customarily in the form of a con- Cal present invention this object is effected by con-- trol grid of the device or devices, the relative desired unidirectional operating potential between the two electrodes may be maintained. For example, by grounding the grid return directly rather than through the cathode, flow of high frequency currents through the heating leads is still further effectively prevented. In other words, my present arrangement puts all heated cathode elements and grid return points at substantially the same reference or Zero potential with respect to high frequency or undulatory currents.

I have found that with my improved heating system, because of the prevention of feed back at high radio frequencies, it is possible to obtain greater amplification with greater freedom from self oscillation than has been possible with high frequency circuits used heretofore.

My present invention is described more fully with the aid of the accompanying drawing illustrating a superheterodyne receiving system, although it is to be clearly understood that my present invention is not limited thereto but may be applied equally well to other apparatus such as high frequency transmitters or to any forr i of repeating systems.

Turning to the drawing, high frequency signaling energy collected upon an antenna 2 is fed to a high frequency amplifier 4 and 6 and then to a first detector 14. The first detector 14. is supplied through a conductor 8, with amplified, locally generated oscillatory energy from an amplifier 10 in turn supplied with heterodyning oscillatory energy from local oscillator 12. The beat frequency energy appearing in the output circuit of first detector 14 is fed through a low pass filter 16 to an intermediate frequency amplifier and second detector 18 which demodulates the intermediate frequency energy in order to obtain the signal which is, in turn, transmitted to an audio frequency or low frequency amplifier 19 energiz- 9 ing a translating device such as ear phones 20. In practice it has been found desirable to use for amplifier 6 and amplifier l0 heater type of tubes or, in other words, tubes having cathodes comprising a heated element in the form of an electron emitting cathode surface 22, and a heater element in the form of a heating filament 24.

In accordance with my present invention, to conserve upon heating current, the filaments or heater elements 24 are connected serially by means of conductors 26, 28 and supplied with unidirectional heating energy from a source 30.

To prevent the flow of high frequency undulatory electrical energy through the heater leads 26, 28 which would tend to cause self oscillation of the system as a whole and undesired parasitic oscillation, each of the electron emitting cathodes 22 is connected to ground through a path 32 of low impedance comprising, as indicated, a relatively large condenser. In addition, to prevent any possibility of fiow of undulatory current from the cathodes to the heater elements, the cathodes and heater elements are connected together in accordance with my present invention by paths 34 having high impedance to undulatory currents comprising, as indicated, radio frequency choke coils 34.

To prevent the interflow of high frequency current from the tunable input circuit 36 of amplifier 6 into the heating circuit, the cold electrode or grid to which the tunable circuit 36 is connected, is grounded for unidirectional potentials, as indicated, through the coil forming part of the tunable input circuit 36. However, in order to maintain the control grid of tube 6 at a suitable operating potential relative to the cathode electron emitting surface 22, the heated element 22 is connected through choke coil 34 to a point 38 of suitable unidirectional potential on the cathode heater circuit. For adjustment of the potential at which the electron emitting cathode 22 is maintained, a resistance 40 completing the cathode heater circuit to ground is provided. Additional resistor 42 in series with the source 30 also facilitates this adjustment.

Similarly, the control grid of tube 10 may be maintained at a suitable operating potential by the combined action of the grid leak resistor 44 and blocking condenser 46 together with the choice of a suitable tapping point 48 for the choke coil 34 associated with the cathode of tube 10.

In this manner, the cathodes of tubes 6, 10, are isolated with respect to radio frequency currents by virtue of the choke coils 34 and condensers 32 and simultaneously, the choke coils maintain the electron emitting cathodes at suitable unidirectional operating potentials with reference to the other electrodes of the respective electron discharge devices.

This iorm of cathode heating system may be used in the radio frequency amplifier stages 4 if found desirable. Inasmuch as the corresponding circuits would require for the greater part simply the repetition of the connections shown for tubes 6, 10, and inasmuch as the connections from the foregoing description would be apparent to one skilled in the art, they have not been given here.

As it may be found desirable to have a very delicate control for the heating current of the local oscillator 12, this current may be brought in from a source 50 through a ballast lamp 52, well known in the art, and through a choke 54 to the filament of tube 12. Flow of radio frequency currents through the ballast lamp and the leads associated therewith is, of course, prevented by the choke 54 and the by-passing condenser 56 connected across the terminals of the filament for tube 12.'

By virtue of the fact that condenser 56 grounds the midpoint of tunable circuit 58 of the local oscillator 12, the filament thereof is effectively connected for radio frequency currents to that point. For fine adjustment of the frequency of oscillations generated a Vernier condenser 60 is provided which is efiectively connected across a portion of the coil forming the tunable circuit 58.

Similarly, the energizing current for the oathode of detector 1% may be supplied independently through variable resistor 62 if found desirable.

Having thus described my invention, what I claim is:

1. In a system comprising a plurality of electrically related electron discharge devices each having an electron emitting cathode, and a filament for heating said electron emitting cathode, means for supplying heating energy serially through said filaments, and, means for preventing the fiow of undulatory electrical currents from said electron emitting cathodes to said heating elements said last mentioned means comprising a choke coil connected between each of said cathodes and each of said filaments, and, a condenser connected from each of said electron emitting cathodes to ground.

2. In apparatus of the character described, an electron discharge device having an anode, a cathode, and a grid, said cathode comprising a heated element and a heater element, means for supplying unidirectional electrical energy to said heater element for heating said heated element, a connection having relatively high impedance to undulatory electrical currents connected from said heated element to said heater element for suitably polarizing said heated element relative to said grid, 2. connection of relatively low impedance to undulatory currents from said heated element to ground, and means for providing a path, of relatively low resistance to unidirectional current, from said grid to ground and a unidirectional current path between said heater element and ground, whereby said grid is maintained at a suitable operating potential relative to said electron emitting heated element.

3. In a thermionic relaying device, an electron discharge tube having an envelope, a control electrode therein, an electron emitting cathode therein, a filament for heating said electron emitting .cathode, a direct current connection between said filament and ground, means for supplying direct current heating energy to said filament, means for maintaining said control electrode at substantially ground direct current potential, and means for preventing interchange of undulatory energy between said electron emitting cathode and said filament and for maintaining said electron emitting cathode at a potential difierent than the potential of said control electrode comprising, an inductive impedance connected directly between said electron emitting cathode and said filament, and a reactive impedance connected directly between said electron emitting cathode and ground.

JOHN B. MOORE. 

